1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor and an apparatus using the method, and more particularly, to a method of arranging mask patterns and an apparatus using the method.
2. Description of the Related Art
One of the goals in integrated circuit fabrication is to faithfully reproduce the original circuit design on a semiconductor wafer by using as much area of the semiconductor wafer as possible. Another goal is to optimize exposure and improve image intensity on the semiconductor wafer. Yet another goal is to increase the depth of focus (DOF) and exposure latitude (EL). However, the microscopic size of main features makes it difficult for light to pass through holes or lines in the photomask used to transfer such features to the wafer. Consequently, the DOF and the EL are reduced.
Conventional methods suggested in order to solve this problem include a method of placing assist features in a mask such that light intensity on a feature to be generated can be increased (which, in turn, will increase DOF and EL). It is known that proper use of assist features generally results in improvements in processing performance. However, no methodology on how to specifically place assist features in order to obtain optimal processing performance, and in a particular processing performance such as EL, DOF, or a mask error enhancement factor (MEEF), has been established. In particular, DOF performance is maximized when the difference in imaging performance at two or more focal positions is minimized. Therefore, optimization of the DOF performance requires more complex calculation than that of EL or MEEF which is performed under a single processing condition. Recently, exposure equipment having a very high numerical aperture (NA) value is being increasingly used in lithography processes. This has resulted in a sharp reduction in DOF performance. Therefore, the enhancement of processing performance, particularly the DOF performance, is much desired.
Conventional method places assist features based on an imaging result at a defocus position, thereby enhancing imaging performance at the defocus position and thus securing a wider DOF. However, since this method completely ignores information regarding an optimal focus position, DOF optimization cannot be guaranteed. This is because DOF performance is maximized when the difference between imaging performance at the optimal focus position and imaging performance at the defocus position is minimized.
Therefore, a new method of effectively determining optimal positions of assist features is required in order to obtain maximum DOF performance.